The present invention relates to touch sensors or touch screens. More particularly, the present invention relates to a spacer adhesive material for touch sensors or touch screens.
Generally, touch sensors or touch screens, such as, capacitive or resistive touch screens, are used in front of a computer driven display capable of variable images or in front of a non-variable display capable of providing fixed images. The touch sensor or touch screen provides an interface so that a human can provide commands to a computer or other control device. Touch screens can be used with computers, control panels, controllers, pocket organizers (e.g., PALM(trademark) handheld computer commercially available from Palm, Inc. of Santa Clara, Calif.), arcade games, or any electronic device requiring human interaction. Generally, the touch screen is placed above (in front of) the display and includes at least one electrically conductive layer, which is used to sense the presence and location of a touch.
As an example of one type of touch screen, a conventional resistive touch screen includes two layers which are often referred to as a flex layer and a stable layer. Both the flex layer and the stable layer have transparent conductive coatings on opposing surfaces. A spacer material (or materials) separates the flex layer and the stable layer from each other. The spacer material ensures that an air gap or other relatively non-conductive medium separates the conductive coatings when the touch screen is not touched or depressed.
When the outer front surface of the touch screen is deformed or pressed, the two transparent conductive coatings are brought into electrical contact. More specifically, the flex layer is deformed and the conductive coating on the flex layer contacts the conductive coating on the stable layer. Typically, the stable layer is not flexible.
Conventional resistive touch screens include matrix touch screens and analog touch screens. Matrix touch screens generally have transparent conductive coatings patterned in rows on one surface (e.g., the flex layer) and in columns (orthogonal to the rows) on the opposing surface (e.g., the stable layer). When force is applied and electrical contact is made (as described above), a discrete switch is closed. The discrete switch is associated with a particular row and column. A computer or other electronic circuit can be used to provide electric signals from the rows and columns to determine the horizontal and vertical position (X, Y coordinate) associated with the discrete switch that is closed.
In analog resistive touch screens, the transparent conductive coatings are provided on the flex and stable layers and are often an indium tin oxide (xe2x80x9cITOxe2x80x9d) material. The conductive coatings have uniform sheet resistivity. The sheet resistivity used in analog resistive touch screens is typically between about 100 and 1000 Ohms/square, with about 200 to 400 Ohms/square being a more preferred resistivity.
A voltage is applied to one end of one of the transparent conductive (resistive) layers through a conductive bus bar, while the bus bar at the other end of the same layer is held at ground. This produces a linear voltage gradient across the screen. The bus bar is configured to create a horizontal voltage gradient on one screen (e.g., flex layer) and a vertical voltage gradient on the other screen (e.g., stable layer). When a force, such as, by an input device (e.g., finger, stylus, etc.) is applied to the flex layer, the flex layer electrically contacts the stable layer and the switch is closed. With the switch closed, one floating layer (e.g., flex layer) is used to receive the voltage created by the gradient on the other layer (e.g., stable layer) at the point of contact. The role of each layer is then reversed and the voltage is measured on the other layer. The analog resistive touch screen may be connected to a computer or electronic circuit that decodes the voltages and converts them to a position associated with the touch. Two voltage readings are used to assign a horizontal and vertical position or point (X, Y coordinate) for the location of the touch. Points can be recorded electronically so rapidly that signatures can be digitized and recorded.
Conventional touch screens generally utilize a spacer material including an acrylic pressure sensitive adhesive (xe2x80x9cPSAxe2x80x9d) to hold the flex layer and the stable layer together and to space apart the flex layer and the stable layer. The spacer material is typically 0.001 to 0.010 inch thick adhesive. The spacer adhesive is typically cut from pre-coated rolls or sheets of adhesive with a release liner on both sides. The spacer material is typically adhered to only the perimeter of the flex layer or the stable layer; the center of the spacer material is left open so that the flex layer can make contact to the stable layer when pressure is applied.
Typically, acrylic PSA is used as the spacer material. The acrylic PSA may or may not be provided with a thin plastic support, such as, a 0.001 inch polyester layer. The sheets are typically stack cut to the appropriate size and die cut with a steel rule die. Most resistive touch screens are rectangular shaped, and therefore the desired spacer adhesive is a rectangular ring.
Generally, substantially more material is discarded than is actually used during fabrication or manufacture of the touch screen. The waste associated with these cutting operations is typically removed by hand and discarded. Accordingly, waste associated with the conventional spacer material is large. In addition, the manual processes associated with the removal of waste adds to the expense of manufacturing the touch screen. Further, the cutting and removal operations associated with conventional spacer materials creates debris that can adversely affect the optical quality associated with the touch screen.
Most acrylic PSA used in conventional spacer materials can have a substantial adverse effect on the resistivity of the conductive coatings (e.g., ITO), especially in high temperature and high humidity environments. Thus, conventional acrylic PSAs are not compatible with the conductive coatings used in touch screens.
Silicone-based PSA sheets are also available. However, such silicone-based PSA sheets are disadvantageous in touch screen applications because they can cost approximately seven times the cost of acrylic PSA sheets.
Screen printed, ultra-violet light cured acrylate adhesives have rarely been used with touch screens due to inadequate performance. UV cured acrylate adhesives are made from a mixture of various acrylate monomers and oligomers to produce a relatively low glass transition temperature (Tg) coating, which is UV cured to a pressure sensitive solid. The cured adhesive can be covered with a silicone release liner. The UV cured acrylate adhesives are susceptible to foaming during printing, show adhesive creep into the electrically active area, are difficult to reproduce, and adversely affect the conductivity of ITO. In addition, these adhesives have poor holding characteristics at elevated temperatures (e.g., the flex layer xe2x80x9cde-bondsxe2x80x9d, comes apart or separates from the stable layer at high temperatures).
Acrylic PSAs have been applied by screen printing in certain less demanding applications than touch screen applications. However, the polymers used with solvent-based acrylic PSAs make screen printing difficult. For example, acrylic PSAs are susceptible to foaming and stringy rheology. In addition, acrylic PSAs tend to have too low of a solids content, which may not sufficiently achieve the desired final dry thickness. Further still, acrylic PSAs are only typically available in relatively fast evaporating solvents, which tend to xe2x80x9cplugxe2x80x9d the print screen.
Thus, there is a need for an adhesive material that is compatible with conductive coatings. Further, there is a need for a method of applying a spacer material for a touch screen that does not create significant debris. Further still, there is a need for a low cost adhesive material that can be easily applied in a touch screen application. Even further still, there is a need for a touch screen that is relatively easy to assemble and includes a spacer material that does not react substantially with conductive coatings associated with the flex and stable layers. Further still, there is a need for a printed adhesive with the mechanical integrity to hold touch screens together in adverse environments where other printed adhesives fail.
An exemplary embodiment relates to a pressure sensitive spacer coating for a touch screen. The pressure sensitive spacer coating includes a silicone adhesive material dissolved in a solvent. The silicone adhesive material can include at least one of polydimethyl siloxane, polydiphenyl siloxane and polydimethyldiphenyl siloxane.
Another exemplary embodiment relates to an adhesive mixture. The adhesive mixture is configured for applying to a surface of a touch screen as a spacer adhesive. The mixture includes a silicone pressure sensitive adhesive and a relatively slow evaporating solvent. The solvent can include at least one of toluene and xylene.
Another exemplary embodiment relates to a touch screen. The touch screen includes a first layer and second layer. The first layer includes a first conductive coating, and the second layer includes a second conductive coating. A silicone pressure sensitive adhesive is disposed between the first layer and the second layer.
Yet another exemplary embodiment relates to a method of manufacturing a touch screen. The method includes providing a first layer having a first translucent conductive surface, providing a second layer having a second translucent conductive surface, and applying an adhesive solution including silicone on at least one of the first conductive surface and the second conductive surface.